The batteries should last a very long time. The best thing is that the performance of the phono doesn't deteriorate as the battery loses charge. This was a concern of mine which I clarified with Ray. Basically, the phono stage will turn off if the supplied power is not optimum for perfect operation. There is no loss in sound quality with the battery discharging. If it is working, it is working perfectly OR it turns off.
If the batteries ever need replacing in a few years of heavy usage, send the Nighthawk back to Ray for a quick turnaround. He'll replace the batteries for you and you are back up and running in no time. This is a small quirk to deal with for the pitch black background of the Nighthawk. I love the convenience of a server based computer audio setup, however, I still deal with the quirks of vinyl because it's all about the sound in the end for me :) Double the quirks with those 45rpm discs where I have to get up ever 10 minutes or so to change the side ;)